1. Technical Field
The invention relates to nuclear fuel assemblies for use in water cooled and moderated reactors, of the type having a bundle of fuel rods each with a zirconiumbase alloy sheath, held in position in a regular pattern by spacing grids distributed along the lengths of the rods and belonging to a structural skeleton of the assembly.
The fuel rods include a stack of uranium and/or plutonium oxide sintered pellets occupying part of the length of the sheath, closed by two plugs of zirconium base alloy.
During operation of a reactor containing a fuel assembly of the above-defined type, the sheaths of the rods (as well as the other zirconium base alloy member, to a lesser degree) exhibit corrosion. The surface exposed to the water is gradually covered with a layer of zirconium oxide ZrO.sub.2 coming from the reaction: EQU Zr+2H.sub.2 O.fwdarw.ZrO.sub.2 +2H.sub.2
This reaction has two consequences. The presence of hydrogen in the zirconium alloy causes the formation of hydrides whose content must be limited for avoiding brittleness. The presence of the zirconia layer, of low heat conductivity, increases the resistance opposed to the flow of heat released by the fuel material so that, at a given coolant temperature, the temperature of the fuel and of the metal part of the sheath increases. The problem is aggravated by the fact that the kinetics for the formation of ZrO.sub.2 increase rapidly with the temperature at the zirconium alloy-zirconia interface and with the diffusion of oxygen anions and oxydrile as far as the interface. As for the formation of hydrides, it depends mainly on the temperature of the metal and on the diffusion of the hydrogenated species as far as the metal.
2. Prior Art
It has already been proposed to protect nuclear fuel sheaths against oxidation by coating them with chromium deposited by chemical evaporation process under vacuum, by ion implementation or by spraying (French Pat. No. 2,317,372). Such chromium coatings are not fully satisfactory.
British Pat. No. 992,677 describes a process for enhancing the corrosion resistance of sheaths, for instance of zironcium, consisting in superficially enriching the sheath with an element which can be carbon. However, the reference fails to provide data sufficient to select appropriate elements and amounts.